In high power applications, a parallel connection of inverters can be the most cost effective way of implementation. A plurality of parallel-connected inverters can be arranged to synchronously supply a common load or grid, for example.
Even if the output voltages produced by the parallel-connected inverters are synchronous, circulating currents can appear. The inverters generate output voltages by controlling switches in their inverter bridges to different operating states, e.g., to conducting or non-conducting states. Modulators in the inverter control the operating states to form switching patterns. By controlling the duty ratios of the switching patterns during switching cycles of the modulators, desired output voltages can be produced.
Each modulator of a parallel-connected inverter can form switching patterns on the basis of a locally generated switching frequency. The switching frequencies of the modulators can differ from each other or have a phase shift between each other, which can lead to a phase shift between the switching patterns of generated by the modulators. Thus, the corresponding switches in parallel-connected inverters can be in different operating states. As a result, paths for circulating currents through the switches in different operating states can appear. A circulating current flowing on such a path can cause undesirable strain to the inverter and the output filter.
A circulating current invoked by a phase shift between modulators can appear in parallel-connected inverters having their DC links tied together, for example. FIG. 1a shows an exemplary arrangement where two inverters 11 are connected in parallel and share a DC link 12. The inverters 11 supply a load 13 through output filters 14. The output filters can be LCL filters, for example. The DC link 12 completes a path of circulating current through the inverters 11 and the output filters 14.
A circulating current can also appear in applications where each inverter can include a DC link. In solar parks, for example, parallel-connected inverters can be separate units that are used for transforming the produced electric energy. The units can each have their own output filters and they can be located at a long distance away from each other.
FIG. 1b illustrates an arrangement where four parallel-connected solar inverters 15 supply a medium voltage transformer 16. The DC links of the solar inverters 11 are not directly connected to each other. However, the DC links can be coupled to each other through ground capacitances formed by the solar panels and their grounded frames. Thus, the grounding between the panels can complete paths for circulating currents.
Circulating currents can be limited (e.g., reduced or elimated) by using communication links between the parallel-connected inverters for synchronizing the inverters. The circulating currents can also be limited by using common-mode current filters. However, the additional components specified for varying forms of the communication links or the current filtering can be expensive and/or bulky.
Finnish patents FI116649 and FI116105 describe methods based on measuring the circulating current and then synchronizing the parallel-connected inverters on the basis of the current measurement. However, additional current sensors can be used for these approaches.